The rear brakes of tractors of tractor-trailer vehicles are designed to accommodate the braking requirements of a fully loaded tractor-trailer combination, commonly referred to as the "trailering mode." Consequently, tractor rear brakes are comparatively oversized when the tractor is operated without the trailer, commonly referred to as the "bobtail mode." As a result, during bobtail operation, oversized tractor rear brakes typically promote premature rear brake lock, which can cause skidding, as well as poor vehicle stability and control.
Various solutions have been proposed in order to alleviate or minimize this problem. Anti-lock brake control systems can provide skid control. However, such skid control systems are expensive and require additional hardware and maintenance.
Proportioning modules are commonly used in the brake control systems of tractors. Proportioning modules generally operate to proportion brake service pressure between the front and rear tractor brakes to prevent premature rear brake lock and its attendant problems. Proportioning modules are disclosed, for example, in U.S. Pat. Nos. 4,410,218, 4,571,007, and 4,725,101. The disclosed proportioning modules override proportioning during bobtail operation when equal front-to-rear brake pressures are desirable, such as, for example, in an emergency created as a result of a failure of the tractor front brake circuit. Proportioning modules typically utilize primary proportioning means that are responsive to service pressure only. As a result, the disclosed modules typically require excessive force to operate the valve to provide desired proportioning ratios or "cut-back" features. Further, due to the high actuating forces required to operate the proportioning means, these modules are generally more complex and have larger packaging sizes than are desirable. Additionally, because the disclosed proportioning modules comprise proportioning means that are responsive to only service pressure, the potential operational response and relief curves are generally quite limited.